Synthesis and thermophysical properties of imidazolate-based ionic liquids: Influences of different cations and anions

Accepted Manuscript Synthesis and thermophysical properties of imidazolate-based ionic liquids: Influences of different cations and anions Yi Zhang, Ting Li, Zaikun Wu, Ping Yu, Yunbai Luo PII: DOI: Reference:

S0021-9614(14)00038-X http://dx.doi.org/10.1016/j.jct.2014.01.028 YJCHT 3840

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

22 April 2013 26 January 2014 31 January 2014

Please cite this article as: Y. Zhang, T. Li, Z. Wu, P. Yu, Y. Luo, Synthesis and thermophysical properties of imidazolate-based ionic liquids: Influences of different cations and anions, J. Chem. Thermodynamics (2014), doi: http://dx.doi.org/10.1016/j.jct.2014.01.028

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and thermophysical properties of imidazolate-based ionic liquids: Influences of different cations and anions Yi Zhang, Ting Li, Zaikun Wu, Ping Yu and Yunbai Luo* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China Abstract Six

novel

imidazolate-based

1-butyl-3-methylimidazolium

room-temperature

imidazolate

([Bmim][Im]),

ionic

liquids

(ILs),

1-ethyl-3-methylimidazolium

imidazolate ([Emim][Im]), 1-hydroxylethyl-3-methylimidazolium imidazolate ([HO-emim][Im]), 1-aminopropyl-3-methylimidazolium 1,4-Bis(3-methylimidazolium-1-yl)butane

imidazolate imidazolate

([NH2-pmim][Im]),

([Bis(mim)C4][Im]2)

and

1,2-Bis(3-methylimidazolium-1-yl)ethane imidazolate ([Bis(mim)C2][Im]2), were prepared with different kinds of cations, including conventional monocation, functionalized cation and dication. Their main physicochemical properties were measured, consisting of glass transition temperature, density, conductivity and viscosity. The influences of the cationic structure on each property were highly discussed. The results showed that the glass transition temperature increased with the decreasing alkyl chains length of cation and dication, whereas the density ∗

Corresponding author. Tel: +86-27-68752511. E-mail address: [email protected] (Y.B. Luo).

1

exhibited an opposite behavior. The viscosity of dicationic and functionalized ILs increased compared to conventional monocationic ILs. However, the conductivity was found to decrease after using dication and functionalized cation. In addition, the density and conductivity of imidazolate ILs were determined as the functions of temperature from T= (293.15 to 343.15) K. Vogel–Tammann–Fulcher and Arrhenius equations were utilized to study the temperaturedependence of conductivity. Moreover, the thermal expansion coefficients were calculated from the density-temperature dependence, the relationship between viscosity and conductivity was discussed using Walden plots. Key Words Ionic liquids; thermo physical properties; correlation. 1. Introduction Ionic liquids (ILs) are organic molten salts, and their melting points are usually below the room temperature. Compared to conventional organic solvents, their unique properties, including low melting points, negligible volatility, high thermal stability, adjustable ions, and high CO2 solubility, make them good candidates as green and designed solvents. Therefore, ILs have been extensively studied in different areas, such as electrolyte bases [1, 2], environmentally friendly solvents, catalysis in organic reaction [3, 4], reversible agents for ion extraction [5, 6], powerful solvents for macromolecules [7, 8] and absorbents for removing CO2 [9, 10]. During the recent decade, ILs are generally formed by combining a diverse range of cations, such as imidazolium, quaternary ammonium, phosphonium and pyridinium, together with tetrafluoroborate ([BF4]-), hexafluorophosphate ([PF6]-), trifluoromethanesulfonate ([TFO]-) and bis(trifluoromethylsulfonyl) amide ([Tf2N]-) anions [11-15]. Very recently, in order to satisfy the further applications of ILs to many important process, functionalized ILs with suitable properties

2

are prepared by introducing amino group, carboxylic group, amino acid group and any other functional groups onto the structure of ILs. For instance, Zhang and Wu’ groups respectively synthesize several dual and triple amino functionalized phosphonium ionic liquids, (3-aminopropyl)

tributylphosphonium

amino

acid

salts

([aP4443][AA])

and

N-(3-aminopropyl)aminoethyl tributylphosphonium amino acid salts ([apaeP444][AA]), and then they are used in chemical absorption of CO2 because of their multiply amino structures [16, 17]. On the other hand, dicationic ILs (DILs) have received considerable attentions due to their superior physical properties compared to monocationic ILs in terms of thermal stability and volatility. And they have been applied in electrochemistry, organic synthesis and biocatalysis. For

example,

several

imidazolium

and

tetraalkylammonium-based

symmetrical

and

asymmetrical DILs are used as electrolyte additives by Zhang, due to their high cathodic stability toward lithium [18, 19]. And the DILs based on pyrrolidinium with [Tf2N]- are demonstrated as the optimum solvents in the Claisen rearrangement and the Diels-Alder reactions at high-temperature because they have high thermal stabilities during reactions [20]. Moreover, molten dicationic imidazolium chloride or bromide salts are proved to be the stable and reusable medias for the controlled pyrolysis of cellulose without acid pretreatment to give anhydrosugars at a lower temperature [21]. Because the acetate ionic liquids can react with CO2 and exhibit the striking CO2 capacity in absorption, we choose imidazolate as the novel anion to improve the CO2 capacity due to the higher basicity of imidazolate (pKb=-0.5) compared to acetate (pKb=9.25). In this study, we presented a scheme to prepare a serial of novel imidazolate-based ILs respectively paired with six different imidazolium-based cations, consisting of conventional monocation, functionalized cation and dication. As illustrated in Figure 1, six different cations and imidazolate were

3

available. For conventional monocationic ILs (CILs), the lengths of the alkyl side-chain substituted at the third position of the imidazolium ring were fixed at two or four carbons. For two functionalized monocationic ILs (FILs), amino and hydroxyl functional groups were incorporated onto the imidazolium-based cation, respectively. Different from monocationic ILs, the dicationic ILs were formed with two head groups linked by a two or four flexible alkyl chain and two imidazolate anions. Since these imidazolate-based ILs are new compounds, the detailed physical properties are of great importance for future applications in many areas. Therefore, the study was designed to measure the physicochemical properties, including glass transition temperature, density, thermal expansion coefficients, conductivity and viscosity. In addition, these properties variations that corresponded to the particular anion and cation structures will be determined. Moreover, the effects of temperature on some properties, the inter relationship between viscosity and conductivity were both discussed, respectively. 2. Experimental 2.1. Chemicals All chemicals (AR degree) were obtained from Sinopharm Chemical Reagent Co., Ltd, China,

except

for

1-hydroxylethyl-3-methylimidazolium

chloride

and

1-amine-proply-3-methylimidazolium bromide (Lanzhou Greenchem ILS, LICP. CAS. China, Purity > 99%). The studied ILs were all synthesized in the laboratory, the water contents of which were measured by Karl Fischer titration analysis within at most 0.5 mass%. The halide content of each sample was determined by ion chromatograph, with a maximum remain content of 0.2 mass%. Therefore, the purity of our studied ILs was above 99%. The structures of

4

prepared ILs were identified by a NMR spectrometer (Varian Mecury VX-300) and a FTIR Spectrophotometer (Nicolet AVATAR 360). 2.2. Synthesis The monocationic and dicationic bromide salts were synthesized according to our previous literatures [22], and characterized with 1H NMR spectra. Besides, all six ILs were synthesized by neutralized equimolar imidazole in ethanol, respectively with different hydroxide salts which were prepared from monocationic and dicationic bromine salts using anion-exchange resin . The mixture was then stirred at room temperature for 12 h. After removing solvent by rotary evaporation and dried in vacuum at 353.15 K for 48 h, the products were obtained. 2.3. Characterization of imidazolate-based ILs 2.3.1. 1-ethyl-3-methylimidazolium imidazolate ([Emim][Im]) Yield: 94.5%. IR: ν= 3149, 3092, 2986, 2938, 2851, 1664, 1571, 1447, 1368, 1169, 1061, 881, 846, 757, 703, 666, 619 cm-1. 1H NMR (300 MHz, DMSO, 25°C) δ 7.68 (d, J = 1.8 Hz, 1H), 7.58 (d, J = 1.7 Hz, 1H), 7.19 (s, 1H), 6.73 (s, 2H), 4.08 (q, J = 7.3 Hz, 2H), 3.74 (s, 3H), 1.34 (t, J = 7.3 Hz, 3H). 2.3.2. 1-butyl-3-methylimidazolium imidazolate ([Bmim][Im]) Yield: 89.3%. IR: ν= 3150, 3092, 2961, 2938, 2851, 1661, 1596, 1449, 1376, 1169, 1062, 881, 829, 754, 705, 665, 621 cm-1. 1H NMR (300 MHz, DMSO, 25°C) δ 7.72 (d, J = 1.7 Hz, 1H), 7.63 (d, J = 1.7 Hz, 1H), 7.21 (s, 1H), 6.75 (s, 2H), 4.07 (t, J = 7.2 Hz, 2H), 3.75 (s, 3H), 1.76 – 1.62 (m, 2H), 1.21 (dd, J = 15.0, 7.4 Hz, 2H), 0.86 (t, J = 7.3 Hz, 3H). 2.3.3. 1-hydroxylethyl-3-methylimidazolium imidazolate ([HO-emim][Im])

5

Yield: 90.0%. IR: ν=3159, 3103, 2959, 2924, 2853, 1663, 1620, 1572, 1528, 1448, 1371, 1253, 1167, 1064, 1021, 835, 756, 690, 665, 644, 620 cm-1. 1H NMR (300 MHz, DMSO, 25°C) δ 7.68 (d, J = 1.7 Hz, 1H), 7.61 (d, J = 1.5 Hz, 1H), 7.17 (s, 1H), 6.72 (d, J = 0.6 Hz, 2H), 4.22 – 4.09 (m, 2H), 3.77 (s, 3H), 3.75 – 3.65 (m, 2H). 2.3.4. 1-aminepropyl-3-methylimidazolium imidazolate ([NH2-pmim][Im]) Yield: 94.7%. IR: ν= 3147, 3091, 2961, 2932, 2850, 1652, 1633, 1592, 1446, 1403, 1167, 1063, 880, 846, 831, 759, 704, 667, 622 cm-1. 1H NMR (300MHz, DMSO, 25°C): 7.70 (s, 1H), 7.63 (s, 1H), 7.21 (s, 1H), 6.73 (s, 2H), 4.14 (t, 2H), 3.75 (s, 3H), 2.43 (m, 2H), 1.76 (t, 2H). 2.3.5. 1,2-Bis(3-methylimidazolium-1-yl)ethane imidazolate ([Bis(mim)C2][Im]2) Yield: 74.6%. IR: ν=3149, 3092, 2988, 2939, 2853, 1664, 1577, 1447, 1367, 1170, 1061, 881, 846, 758, 703, 665, 620 cm-1. 1H NMR (300 MHz, DMSO, 25°C) δ 7.61 (s, 2H), 7.54 (s, 2H), 7.31 (s, 2H), 6.80 (s, 4H), 4.57 (s, 4H), 3.74 (s, 6H). 2.3.6. 1,4-Bis(3-methylimidazolium-1-yl)butane imidazolate ([Bis(mim)C4][Im]2) Yield: 76.6%. IR: ν=3146, 3090, 2990, 2940, 2851, 1572, 1450, 1374, 1166, 1060, 881, 833, 756, 703, 665, 619 cm-1. 1H NMR (300 MHz, DMSO, 25°C) δ 7.75 (d, J = 1.4 Hz, 2H), 7.68 (d, J = 1.7 Hz, 2H), 7.36 – 7.22 (m, 2H), 6.85 – 6.75 (m, 4H), 4.14 (s, 4H), 3.81 (d, J = 1.1 Hz, 6H), 1.71 (s, 4H). 2.4. Properties measurements of synthesized ionic liquids 2.4.1. Glass Transition Temperature Measurements The glass transition temperature was determined by the differential scanning calorimeter (Mettler-Toledo DSC822e). Because the volatiles evidently affected the glass transition

6

temperature, each sample was dried in vacuum at 353.15 K for 48 hours. And the uncertainty was determined by the known ILs within at most ±1 K. 2.4.2. Density measurements Because of the limited amount of ILs, the density of ILs was measured with a 5mL pycnometer in the temperature range (293.15 to 343.15) K at atmospheric pressure and repeated for three times. Before measuring, the pycnometer was calibrated using standard oil and validated using the other familiar ILs at certain temperatures. 2.4.3. Conductivity Measurements The conductivity of ILs was measured with microprocessor conductivity meter (DDS-12DW) from (293.15 to 343.15) K. Each value of the conductivity was the average of three measurements with at least ±1% reproducibility. Before measuring the conductivity data of the ILs, the precision of the analyzing instrument was determined with the sample of the known ILs. 2.4.4. Viscosity Measurements The viscosity were determined by capillary viscometers at different temperatures ranging from (293.15 to 343.15) K. Viscosity was collected by measuring the efflux time required for liquid to flow between marks on the viscometer and repeated three times with a reproducibility below ±1%. Before measuring the viscosity data of the ILs, the reliability of this measurement method has been validated by the two reported ILs, including high and low viscosity ILs. 3. Results and discussion 3.1. Glass transition temperature

7

Table 1 summarized that the glass transition temperatures (Tg) of the studied ILs, which varied from 213.14 to 219.14 K. According to the experimental results, the alkyl side-chain length of the cation and the alkyl link-chain length of the cation both had influences on the Tg values. For example, the Tg of [Emim][Im] was 4 K lower than the value for [Bmim][Im]. And the similar tendency was also observed between [Bis(mim)C2][Im]2 and [Bis(mim)C4][Im]2 with different length of the alkyl link-chain. Moreover, the Tg values of [NH2-pmim][Im] and [HO-emim][Im] were similar to those of [Emim][Im] and [Bmim][Im], which revealed that the Tg was dependent of the main structures of ILs rather than the functionalized substitutional groups. In addition, the imidazolate lowered the melting points of ILs when compared to [Tf2N] and [BF4]

anions.

For

example,

the

diacation

analogous,

[Bis(mim)C2][Tf2N]2

and

[Bis(mim)C4][Tf2N]2 were both the solid powers rather than liquids at room temperature. From the literature [23], we can know that the melting points of [Bis(mim)C4][Tf2N]2 and [Bis(mim)C4][BF4]2 were 332 and 373 K. After replacing the anion with the imidazolate, however, their melting points were below the room temperature. Meanwhile, the Tg values of [Emim][Im] and [Bmim][Im] were much higher than those of the same cationic analogous with [Tf2N] and [BF4] anions, respectively. 3.2. Density The densities of the ILs were showed in Figure 2, which were all measured from T= (293.15 to 343.15) K with 10 K intervals. The density values were found to decrease linearly with an increase of temperature, be higher than those of the water and traditional solvents, such as ethanol, methanol and ethyl acetate [24]. In order to know the influences of anionic structure on densities of the synthesized ILs more directly, several density data of the corresponding ILs were

8

listed in Table 2 and compared with the studied ILs. To specify, the densities of [Emim][Im] and [Bmim][Im] were lower than those of the same cationic analogues, [Cation][X]. Apart from the familiar anions, such as [BF4]- [25, 26], [PF6]- [27], [Tf2N]- [28], the [X]- also included some amino acid anions ([AA]-), such as glycine ([Gly]-) [29], alanine ([Ala]-) [30], propionate ([Pro]-) [30] and aspartate ([Asp]-) [31] . However, the densities of these two imidazolate ILs were a little higher than those of acetate-based ILs ([OAc]-). [32, 33] All these behaviors were in a good agreement with many similar reported conclusions that the densities of ILs increased with the increasing molecular weight of anion. Besides, [NH2-pmim][Im] also had a much lower density than [NH2-pmim][BF4] [34] and [NH2-pmim][PF6] [35]. Whereas, the density of [HO-emim][Im] was very close to that of [HO-emim][BF4] [36]. In addition, the two imidazolate-based dicationic ILs were compared with the corresponding amino acid-based dicationic ILs in terms of their densities. It was ascertained that their densities decreased in the following order: [Bis(mim)Cn][Gly]2＞[Bis(mim)Cn][Pro]2＞[Bis(mim)Cn][Im]2 [37]. Apart from the influence of anionic structure on the densities, the density variance was also affected by the cationic structures. Because the free volume of the ILs increased, the longer side-alkyl and link-alkyl chains resulted in a decrease in the densities of monocationic ILs and DILs. For example, the densities of [Emim][Im] and [Bis(mim)C2][Im]2 were higher than those of [Bmim][Im] and [Bis(mim)C4][Im]2, respectively. As shown in Table 3, the linear equations of density with temperature were modeled and placed in the Equation 1:

ρ / g ⋅ cm −3 = a + b ⋅ T / K

(1)

Where a, b presented as the parameters for the density equation with temperature T. Taking into account the uncertainty of density, the deviations of the experimental data and the estimated

9

values from the fitting equations were all below ±0.0005 g·cm-3, within the accepted limits. Hence, the model was valid for estimating the density at each temperature. 3.3. Thermal expansion coefficient From the density data, the thermal expansion coefficient could be estimated. As shown in the Equation 2, after ln ρ was plotted against (T-298.15), the linear relationship could be obtained. ln( ρ / g ⋅ cm −3 ) = c − α ⋅ (T / K − 298.15)

(2)

In this equation, c and α stood for the empirical constant and the coefficient of thermal expansion, respectively. Seen from the Table 4, similar to the noticeable difference in their densities, the variation in the thermal expansion coefficient of six ILs was also evident. Because the influences of both functionalized groups and dication exerted on the thermal expansion of ILs were much less compared to the conventional analogues. And the calculated thermal expansion coefficient increased in the following order: [HO-emim] < [NH2-pmim] < [Bis(mim)C4] < [Bis(mim)C2] < [Bmim] < [Emim]. 3.4. Viscosity As showed in Table 5, the viscosities of ILs rapidly decreased with an increase of temperature. Because of a simpler cation with less flexibility of the alkyl chain, the viscosities of [Emim][Im], [Bmim][Im] and [NH2-pmim][Im] were better in fluidity in comparison with the other task-specific ILs. For example, the viscosities of [aP4443][AA] [16] were at least 713.9 mPa·s at 298.15 K. The viscosity of trihexyl(tetradecyl) phosphonium propionate ([P66614][Pro]) [38] was much larger (above 1000 mPa·s at 293.15 K). Furthermore, the viscosities of 1-butyl-3-aminepropyl-imidazolium

tetrafluoroborate

([NH2-pbim][BF4])

[39]

and

10

[apaeP444][AA] [17] were even more higher, because they were in gelatin state at room temperature. One way to improve the CO2 absorption ability of the ILs had been to functionalize them with amino and amino acid group, but it also brought higher viscosity, which would lead to many negative effects on the afterward operations, such as the lower transport between gas and liquid. However, three imidazolate-based ILs were better than conventional amino functionalized ILs in terms of the fluidity. In order to discuss the effects brought about by the structural variations of anions on viscosities, we compared the viscosity of imidazolate-based ILs and some other functionalized ILs, including [Emim][Im], [Emim][Pro], [Emim][Ala] and [Emim][OAc]. The same features of these ILs were that their cations were all the [Emim]+. Seen from the literatures [30], the viscosities of [Emim][Pro] were 626.9, 295.2, 153.6, 87.1, 54.0, 35.5 mPa·s at temperature from 293.15 to 343.15 K with 10 K intervals. And the viscosities of [Emim][Ala] were 235.3, 126.6, 72.0, 45.1, 30.1, 21.2 mPa·s at temperature from 293.15 to 343.15 K with 10 K intervals. The two amino acid-based ILs had much higher viscosities compared to [Emim][Im], because the structures of these two anions were more complicated than that of the imidazolate anion. In addition, the viscosity of [Emim][OAc] were 97.9, 57.2, 36.9, 25.2, 18.1 mPa·s at temperature from 303.15 to 343.15 K, so its viscosity was a little higher than that of [Emim][Im] [40]. After discussing the comparison of viscosity between the studied imidazolate-based ILs and many functionalized ILs which had been applied in CO2 capture, it was also necessary to state the comparisons between all these six imidazolate-based ILs. On one hand, the viscosities decreased with the decreasing length of alkyl chain, so their mobility behaviors of ILs increased. For instance, the viscosities of [Emim][Im] and [Bmim][Im] were 191.8 and 533.3 mPa·s at 293.15 K, respectively. On the other hand, in terms of other three ILs, they all had high viscosity.

11

To start with, the presence of the hydroxyl group significantly increased the number of hydrogen bonds. This phenomenon was also seen in the comparison between the viscosity of 1-methyl-3-octylimidazolium

tetrafluoroborate

([Hmim][BF4])

and

1-hydroxyl-3-methylimidazolium tetrafluoroborate ([HO-hmim][BF4]) that was reported in the past reference [36]. So our study confirmed the opinion that hydroxyl group would brought much higher viscosity compared to its analogues without the hydroxyl group. In addition, the larger size of dication leaded to an increase in the number of van der Waals interactions. Therefore, [Bis(mim)C2][Im]2 and [Bis(mim)C4][Im]2 exhibited obvious high viscosities. In order to avoid the difficulties in the processes of stirring, pumping and mixing, these viscous ILs could be used after mixing with some other low viscosity ILs, such as [Emim][Tf2N] and [Bmim][Tf2N], to increase the mass transport and heat transport. Thereafter the advantages and features of the high viscosity ILs would be partially retained. 3.5. Conductivity As shown in the Table 6, the measured conductivity values dramatically jumped with the increasing temperature. After analyzed with the Vogel–Tammann–Fulcher (VTF) equation, a linear relationship existed between the conductivity and temperature was showed in Figure 3. The VTF function was defined by the following Equation 3. κ / µS ⋅ cm −1 = κ 0 ⋅ (T / K ) −1 / 2 ⋅ EXP{− B /(T / K − Tg / K )}

(3)

In this function, κ0 stood for a pre-exponential constant of ionic conductivity at an infinite high temperature, Tg was the glass transition temperature, and B was the pseudo-activation energy of conduction. On the other hand, Figure 4 illustrated that the linear relationships between ln and (103T-1) were not desirable when the experimental conductivity data were fitted by the Arrhenius equation. This behavior indicated that the temperature dependences of the ionic

12

conductivity for these ILs obeyed the VTF equation rather than the Arrhenius equation. Therefore, the conductivity of ILs exponentially increased with the rise in temperature, and the migration of the carrier ions could be explained by a cooperative transport mechanism [41-43]. Relating to the influence of cationic structure on the conductivity, it was very important to be carefully discussed according to the experimental results. To start with, as the length of the side-alkyl chains of cation increased, the ionic transport rate decreased which leaded to a lower conductivity. For example, at a given temperature, the conductivity of [Emim][Im] was always higher than that of [Bmim][Im]. However, the similar results were not observed between [Bis(mim)C2][Im]2 and [Bis(mim)C4][Im]2. Probably because the [Bis(mim)C2][Im]2 was really too viscous which seriously exerted a negative influence on the conductivity of ILs. Furthermore, the conductivity of [HO-emim][Im] was much lower than other five imidazolate-based ILs, which was also related to its viscous state. Whereas, the amino group did not obviously affect the conductivity of ILs after the comparison between [NH2-pmim][Im], [Emim][Im] and [Bmim][Im]. In addition, we also discussed the effects of different anions. Imidazolate was used to seek for the better properties prefer to the similar anion, acetate. So [OAc]- would be definitely included in the later discussion, and then [AA]- and other conventional anions were also discussed in this study. Taking [Bmim][Im] as an example to be compared with its analogous with different anions. Seen from the literatures, the conductivities of [Bmim][OAc] were 332, 699, 1304, 2213, 3485, 5170 µS·cm-1 at tempearture ranging from 293.15 to 343.15 K with 10 K intervals [44]. And the conductivities of [Bmim][Asp] were 0.82, 1.23, 1.97, 2.78, 3.68 µS·cm-1 at tempearture ranging from 303.15 to 343.15 K with 10 K intervals [31]. Therefore, the conductivity of [Bmim][Im] was higher than [Bmim][OAc] and [Bmim][Asp], which indicated

13

that the ion mobility in [Bmim][Im] was higher. However, the conductivities of [Bmim][BF4] were 3520, 5560, 8210, 11500, 15450, 20000 µS·cm-1 at tempearture ranging from 297.15 to 347.15 K with 10 K intervals [45]. And the conductivities of [Bmim][PF6] were 1465, 2480, 3900, 5780, 8150, 11020 µS·cm-1 at tempearture ranging from 298.15 to 348.15 K with 10 K intervals [46]. Therefore, the conductivity would become lower after using imidazolate instead of the conventional anions. 3.6. Relationship between viscosity and conductivity According to previous work [47, 48], the relationship between the viscosity and the conductivity was depicted by Walden rules. The expression about this rule in our work here was defined as Equation 4. Log (Λ / S ⋅ cm 2 ⋅ mol −1 ) = k ⋅ Log (η −1 / Pa −1 ⋅ s −1 )

(4)

In this function, Λ stood for the equivalent conductivity, η was the viscosity, and k was the constant. As shown in Figure 5, Walden Plots of each ILs over the temperature range 293.15-343.15 K were linear which meant that the molar conductivity-fluidity relationship remained constant and all the studied ILs obeyed the Walden rules. That was, the conductivity value of each studied imidazolate ILs increased with the decreasing viscosity proportionally. In addition, the ideal Walden plot (a line of the 0.01 M KCl solution) was above all the plots, which indicated that all these ILs were positioned into the non-ionic region. In terms of the deviation of each ILs from the ideal Walden plot, it could be obviously seen that the deviation followed this trend for the six ILs with the same [Im] anions: functionalized cation＞monocation＞dication. And according to the statements in the past literatures [49-51], this observation provided that the

14

family of imidazolate ILs exhibited the decreasing “ionicity” trend: dication＞monocation＞ functionalized cation. 4. Conclusions Six imidazolate-based ILs were synthesized and characterized. Glass transition temperatures were determined by DSC scanning. Tg of [Bis(mim)C4][Im]2 and [Bmim][Im] were higher than those of [Bis(mim)C2][Im]2 and [Emim][Im], respectively. The functional groups in cations exhibited negligible effects on Tg. Densities, conductivities and viscosities were all measured at T = (293.15, 303.15, 313.15, 323.15, 333.15 and 343.15) K. As expected, the linear dependences of density and conductivity with the increasing temperature were established to give the calculated data at each temperature. From the relationship of density and temperature, their thermal expansion coefficient was also calculated. Significantly, each property of imidazolate-based ILs typically depended on the cationic structure. In general, the density increased with the decreasing side alkyl chain length of the monocation and the decreasing link alkyl chain length of the dication. Compared to the monocationic ILs, the density and viscosity of functionalized ILs were enhanced, while the conductivity of functionalized ILs showed the opposite trend. On the other hand, the viscosity of DILs was enhanced, but the density and conductivity of DILs decreased after comparing to the monocationic ILs. Finally, the relationship between the viscosity and conductivity was followed as Walden rule, and indicated that “ionicity” for imidazolate-based ILs in this study decreased as the following trend: dication＞monocation＞functionalized cation. Acknowledgements

15

The authors thank the National Science and Technology Support Program (2012BAC02B04) and the Fundamental Research Funds for the Central Universities (FRFCU), China (Award No.201120302020012 and 2012203020213). References [1] U.A. Rana, M. Forsyth, D.R. MacFarlane, J.M. Pringle, Electrochim. Acta, 84 (2012) 213-222. [2] M. Montanino, F. Alessandrini, S. Passerini, G.B. Appetecchi, Electrochim. Acta, 96 (2013) 124-133. [3] J. Ryu, J.-W. Choi, D.J. Suh, D.J. Ahn, Y.-W. Suh, Catal. Commun., 24 (2012) 11-15. [4] Q. Zhao, P. Zhang, M. Antonietti, J. Yuan, J. Am. Chem. Soc., 134 (2012) 11852-11855. [5] V. Zgonnik, C. Zedde, Y. Genisson, M.-R. Mazieres, J.-C. Plaquevent, Chem. Commun., 48 (2012) 3185-3187. [6] Q. Yang, H. Xing, B. Su, K. Yu, Z. Bao, Y. Yang, Q. Ren, Chem. Eng. J., 181–182 (2012) 334-342. [7] H. Wang, G. Gurau, R.D. Rogers, Chem. Soc. Rev., 41 (2012) 1519-1537. [8] M. Moniruzzaman, T. Ono, Biochem. Eng. J., 60 (2012) 156-160. [9] M.B. Shiflett, B.A. Elliott, S.R. Lustig, S. Sabesan, M.S. Kelkar, A. Yokozeki, ChemPhysChem, 13 (2012) 1806-1817. [10] T.K. Carlisle, E.F. Wiesenauer, G.D. Nicodemus, D.L. Gin, R.D. Noble, Ind. Eng. Chem. Res., 52 (2012) 1023-1032. [11] S. Hwang, Y. Park, K. Park, J. Chem. Thermodyn., 43 (2011) 339-343. [12] J. Safarov, I. Kul, W.A. El-Awady, J. Nocke, A. Shahverdiyev, E. Hassel, J. Chem. Thermodyn., 51 (2012) 82-87. [13] G. Vakili-Nezhaad, M. Vatani, M. Asghari, I. Ashour, J. Chem. Thermodyn., 54 (2012) 148-154. [14] V.H. Alvarez, S. Mattedi, M. Aznar, J. Chem. Thermodyn., 62 (2013) 130-141. [15] E. Gómez, N. Calvar, Á. Domínguez, E.A. Macedo, J. Chem. Thermodyn., 42 (2010) 1324-1329. [16] Y. Zhang, S. Zhang, X. Lu, Q. Zhou, W. Fan, X. Zhang, Chem. Eur. J., 15 (2009) 3003-3011. [17] J. Ren, L. Wu, B.-G. Li, Ind. Eng. Chem. Res., 51 (2012) 7901-7909. [18] Z. Zhang, H. Zhou, L. Yang, K. Tachibana, K. Kamijima, J. Xu, Electrochim. Acta, 53 (2008) 4833-4838. [19] Z. Zhang, L. Yang, S. Luo, M. Tian, K. Tachibana, K. Kamijima, J. Power Sources, 167 (2007) 217-222. [20] X. Han, D.W. Armstrong, Org. Lett., 7 (2005) 4205-4208. [21] G.N. Sheldrake, D. Schleck, Green Chem., 9 (2007) 1044-1046. [22] Y. Zhang, Z. Wu, S. Chen, P. Yu, Y. Luo, Ind. Eng. Chem. Res., 52 (2013) 6069-6075. [23] H. Shirota, T. Mandai, H. Fukazawa, T. Kato, J. Chem. Eng. Data, 56 (2011) 2453-2459.

16

[24] R. Seddon Kenneth, A. Stark, M.-J. Torres, Viscosity and Density of 1-Alkyl-3-methylimidazolium Ionic Liquids, in: Clean Solvents, American Chemical Society, 2002, pp. 34-49. [25] A.N. Soriano, B.T. Doma Jr, M.-H. Li, J. Chem. Thermodyn., 41 (2009) 301-307. [26] A.P. Fröba, H. Kremer, A. Leipertz, J. Phys. Chem. B, 112 (2008) 12420-12430. [27] A.B. Pereiro, J.L. Legido, A. Rodrı´guez, J. Chem. Thermodyn., 39 (2007) 1168-1175. [28] C. Kolbeck, J. Lehmann, K.R.J. Lovelock, T. Cremer, N. Paape, P. Wasserscheid, A.P. Fröba, F. Maier, H.P. Steinrück, J. Phys. Chem. B, 114 (2010) 17025-17036. [29] D.H. Dagade, K.R. Madkar, S.P. Shinde, S.S. Barge, J. Phys. Chem. B, 117 (2013) 1031-1043. [30] N. Muhammad, Z.B. Man, M.A. Bustam, M.I.A. Mutalib, C.D. Wilfred, S. Rafiq, J. Chem. Eng. Data, 56 (2011) 3157-3162. [31] Y. Wei, Y. Jin, Z.-J. Wu, Y. Yang, Q.-G. Zhang, Z.-H. Kang, J. Chem. Eng. Data, 58 (2013) 349-356. [32] A.P. Fröba, M.H. Rausch, K. Krzeminski, D. Assenbaum, P. Wasserscheid, A. Leipertz, Int. J. Thermophys., 31 (2010) 2059-2077. [33] M.B. Shiflett, A. Yokozeki, Ind. Eng. Chem. Res., 49 (2009) 1370-1377. [34] K.A. Kurnia, M.I.A. Mutalib, B. Ariwahjoedi, J. Chem. Eng. Data, 56 (2011) 2557-2562. [35] Y. Geng, T. Wang, D. Yu, C. Peng, H. Liu, Y. Hu, Chin. J. Chem. Eng., 16 (2008) 256-262. [36] J. Restolho, A.P. Serro, J.L. Mata, B. Saramago, J. Chem. Eng. Data, 54 (2009) 950-955. [37] Y. Zhang, P. Yu, Y. Luo, Chem. Eng. J., 214 (2013) 355-363. [38] B.F. Goodrich, J.C. de la Fuente, B.E. Gurkan, Z.K. Lopez, E.A. Price, Y. Huang, J.F. Brennecke, J. Phys. Chem. B, 115 (2011) 9140-9150. [39] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, J. Am. Chem. Soc., 124 (2002) 926-927. [40] E. Quijada-Maldonado, S. van der Boogaart, J.H. Lijbers, G.W. Meindersma, A.B. de Haan, J. Chem. Thermodyn., 51 (2012) 51-58. [41] A. Grandjean, M. Malki, C. Simonnet, D. Manara, B. Penelon, Physical Review B, 75 (2007) 054112. [42] G. Gruener, P. Odier, D. De Sousa Meneses, P. Florian, P. Richet, Physical Review B, 64 (2001) 024206. [43] T. Pfeiffer, Solid State Ionics, 105 (1998) 277-287. [44] J.M.M. Araújo, A.B. Pereiro, F. Alves, I.M. Marrucho, L.P.N. Rebelo, J. Chem. Thermodyn., 57 (2013) 1-8. [45] A. Stoppa, O. Zech, W. Kunz, R. Buchner, J. Chem. Eng. Data, 55 (2009) 1768-1773. [46] O. Zech, A. Stoppa, R. Buchner, W. Kunz, J. Chem. Eng. Data, 55 (2010) 1774-1778. [47] K. Hayamizu, Y. Aihara, H. Nakagawa, T. Nukuda, W.S. Price, J. Phys. Chem. B, 108 (2004) 19527-19532. [48] K. Tsunashima, M. Sugiya, Electrochem. Commun., 9 (2007) 2353-2358. [49] K.J. Fraser, E.I. Izgorodina, M. Forsyth, J.L. Scott, D.R. MacFarlane, Chem. Commun., (2007) 3817-3819. [50] D.R. MacFarlane, M. Forsyth, E.I. Izgorodina, A.P. Abbott, G. Annat, K. Fraser, PCCP, 11 (2009) 4962-4967. [51] J.-P. Belieres, C.A. Angell, J. Phys. Chem. B, 111 (2007) 4926-4937.

17

List of Figures FIGURE 1. Chemical structures of the investigated ILs in this work. FIGURE 2. Density of ILs with different temperatures. FIGURE 3. The VTF plots of the conductivity of ILs. FIGURE 4. The Arrhenius plots of the conductivity of ILs. FIGURE 5. Walden plots of log (equivalent conductivity) against log (fluidity).

18

FIGURE 1. Chemical structures of the investigated ILs in this work.

19

FIGURE 2. Density of [Emim][Im] (▼), [Bmim][Im] (▲), [NH2-pmim][Im] (○), [HO-emim][Im] (□), [Bis(mim)C2][Im] 2 (■) and [Bis(mim)C4][Im]2 (●) with different temperatures.

20

FIGURE 3. The VTF plots of the conductivity of [Emim][Im] (▼), [Bmim][Im] (▲), [NH2-pmim][Im] (○), [HO-emim][Im] (□), [Bis(mim)C2][Im]2 (■) and [Bis(mim)C4][Im]2 (●).

21

FIGURE 4. The Arrhenius plots of the conductivity of [Emim][Im] (▼), [Bmim][Im] (▲), [NH2-pmim][Im] (○), [HO-emim][Im] (□), [Bis(mim)C2][Im] 2 (■) and [Bis(mim)C4][Im]2 (●).

22

FIGURE 5. Walden plots of log (equivalent conductivity Λ) against log (fluidity η-1): [Emim][Im] (▼), [Bmim][Im] (▲), [NH2-pmim][Im] (○), [HO-emim][Im] (□), [Bis(mim)C2][Im] 2 (■) and [Bis(mim)C4][Im]2 (●) at T = (293.15 to 343.15) K.

23

List of Tables TABLE 1 Molar weights and glass transition temperatures. TABLE 2 Density data of synthesized and compared ILs at different temperatures. TABLE 3 Fitting parameters and standard deviations for correlating density as a function of temperature. TABLE 4 Thermal expansion coefficients of six ILs at 298.15K. TABLE 5 Viscosities of ILs at different temperatures. TABLE 6 Conductivity of six ILs at different temperatures.

24

TABLE 1 Molar weights M and glass transition temperatures Tg. M / (g·mol-1) 178.18 206.23 207.22 194.18 326.41 354.46

ILs [Emim][Im] [Bmim][Im] [NH2-pmim][Im] [HO-emim][Im] [Bis(mim)C2][Im]2 [Bis(mim)C4][Im]2

Tg / K 213.14 217.14 218.13 219.14 216.97 218.81

TABLE 2 Density data of synthesized and compared ILs at T = (293.15 to 343.15) K. a ILs

a

T/K 293.15 1.1135 1.2828 1.5236 1.1581 1.1392 1.1027 1.0853 1.3716 1.4402 1.1142

[Emim][Im] [Emim][BF4] 29 [Emim][Tf2N] 32 [Emim][Pro] 34 [Emim][Ala] 34 [Emim][OAc] 36 [Bmim][Im] [Bmim][BF4] 30 [Bmim][PF6] 31 [Bmim][Tf2N] 32 [Bmim][Gly] 33 [Bmim][Asp] 35 [Bmim][OAc] 37 [NH2-pmim][Im] [NH2-pmim][BF4] 38 [NH2-pmim][PF6] 39 [HO-emim][Im] [HO-emim][BF4] 40 [Bis(mim)C2][Im]2 [Bis(mim)C2][Gly]2 41 [Bis(mim)C2][Pro]2 41 [Bis(mim)C4][Im]2 [Bis(mim)C4][Gly]2 41 [Bis(mim)C4][Pro]2 41

1.0583 1.1484 1.3580 1.4994 1.3010 1.2108 1.2697 1.2227 1.1774 1.2283 1.2029

303.15 1.1067 1.2776 1.5135 1.1519 1.133 1.0966 1.0799 1.1976 1.3630 1.4307 1.1078 1.1803 1.0517 1.1431 1.3493 1.4926 1.2957 1.3047 1.2060 1.2656 1.2170 1.1728 1.2237 1.1991

313.15 1.1003 1.2725 1.5034 1.1457 1.1266 1.0906 1.0732 1.1908 1.3545 1.4212 1.1016 1.1719 1.0452 1.1375 1.3432 1.4847 1.2898 1.2961 1.2000 1.2598 1.2124 1.1669 1.2177 1.1931

323.15 1.0925 1.2673 1.4935 1.1395 1.1203 1.0845 1.0665 1.184 1.3461 1.4117 1.0955 1.1608 1.0387 1.1318 1.3336 1.4775 1.2837 1.2915 1.1915 1.2524 1.2056 1.1587 1.2140 1.1866

Densities of ILs was measured with g·cm-3, “–” referred to the unnoticed value.

333.15 1.0863 1.2622 1.4836 1.1332 1.114 1.0785 1.0604 1.1772 1.3378 1.4024 1.1519 1.0322 1.1263 1.3254 1.4689 1.2781 1.1857 1.2444 1.2004 1.1543 1.2087 1.1810

343.15 1.0797 1.2571 1.4738 1.1269 1.1078 1.0726 1.0541 1.1704 1.3296 1.393 1.1454 1.0257 1.1208 1.3173 1.4621 1.2723 1.1789 1.2365 1.1948 1.1489 1.2038 1.1765

TABLE 3 Fitting parameters a, b and standard deviations σ for correlating density as a function of temperature. a

a

a / (g· cm-3) 1.3129 1.2710 1.3105 1.4705 1.4036 1.3508

ILs [Emim][Im] [Bmim][Im] [NH2-pmim][Im] [HO-emim][Im] [Bis(mim)C2][Im]2 [Bis(mim)C4][Im]2

104·b / (g·cm-3· K) -6.8002 -6.3214 -5.5270 -5.7744 -6.5425 -5.8968

104·σ / (g· cm-3) 3.6 3.5 0.9 2 9.4 10.1

σ was standard deviations for each sample of the fits:

n SDρ =  Σ ρiexp − ρical i =1

(

2

)

12

(n − v ) . Where n was the number of experimental points and ν was the number of 

adjustable parameters.

25

TABLE 4 Thermal expansion coefficients α of six ILs at 298.15K: densities ρ and empirical constant c. a

a

ρ / (g· cm-3) a 1.1101 1.0826 1.1457 1.2983 1.2085 1.1749

ILs [Emim][Im] [Bmim][Im] [NH2-pmim][Im] [HO-emim][Im] [Bis(mim)C2][Im]2 [Bis(mim)C4][Im]2

104α / K-1 6.20 5.91 4.87 4.49 5.47 5.07

c / (g·cm-3) 0.1045 0.0793 0.1360 0.2611 0.1894 0.1612

Densities of ILs at 298.15 K were calculated by the fitted equation.

TABLE 5 Viscosities η of ILs at T = (293.15 to 343.15) K. “–” referred to the value above 10000 mPa· s. T/K

η / (mPa·s) [Emim][Im]

[Bmim][Im]

[NH2-pmim][Im]

[HO-emim][Im] [Bis(mim)C2][Im]2 [Bis(mim)C4][Im]2

293.15

191.8

533.3

220.0

-

-

-

303.15

87.9

272.6

98.4

8896

6306

3058

313.15

42.4

140

54.5

3632

2538

1003

323.15

22.9

76.8

32.1

1255

854.7

427.1

333.15

14.8

47.5

19.9

615.2

440.2

206.3

343.15

8.6

34.0

13.7

290.4

145.1

111.5

TABLE 6 Conductivity κ of six ILs at T = (293.15 to 343.15) K. κ / (µS· cm-1)

T/K [Emim][Im]

[Bmim][Im]

[NH2-pmim][Im]

293.15

2700

737

842

1.68

111.3

266

303.15

4870

1496

1656

6.36

309.5

686

313.15

8040

2840

3045

22.9

684

1464

323.15

12390

4806

5040

64.6

1453

2835

7129

7375

173.5

2540

4980

10125

10930

391.5

4250

9550

333.15 343.15

16925 21120

[HO-emim][Im] [Bis(mim)C2][Im]2 [Bis(mim)C4][Im]2

26

Hightlights

•

Six novel imidazolate-based ionic liquids are synthesized and characterized.

•

Density, conductivity and viscosity are measured at different temperatures.

•

Glass transition temperatures are determined.

•

Thermal expansion coefficients are calculated.

•

Influences of different cations and anions on each property are discussed.

27